In the past three decades, disk drives have become a standard feature in computer systems. One of the key factors in their popularity has been the availability of smaller disk drives to fit within first minicomputers, then microcomputers, and now notebook computers. One component of the rotating magnetic storage device that has developed during this period is the spindle motor.
The spindle motor rotates the magnetic media and is at the heart of rotating magnetic storage. The first spindle motors were induction motors powered off of ac line power. These motors provided plenty of torque to start and spin the disks and provided a stable rotation rate. However, as markets for computers grew internationally the different line voltages (100 V, 110 V, and 220 V) and frequencies (50 Hz and 60 Hz) in various countries proved a logistical problem. Furthermore, the complications of running the ac line voltage to an internal component of the computer encourages the development of DC voltage powered disk drives.
All DC motors require commutation (switching from energizing one set of motor coils to the next as the motor rotates. The classical way in which this is handled is for the motor to have a commutation ting and brushes. The commutation ring is located on the rotor and the brushes are located on the stator. As the rotor turns, the conductive brushes make contact with different conductive regions on the commutation ring, energizing the proper coil in the rotor to keep the motor spinning in the desired direction. A disadvantage with this DC solution is that the brushes exhibit wear over time. Further, as the brush switches from one coil to another on the commutation ring, there is an arc as the transition is made. The wear processes and the electrical noise make the DC brush motor unsuitable for disk applications.
Another solution for rotating disks is the brushless DC motor. In this case something other than brushes provide the information on when to switch between coils on the motor. One type of brushless DC motor utilizes Hall effect devices which sense a magnetic field. By placing permanent magnets on the rotor, the rotational position of the rotor can be determined and the appropriate coil of the motor energized. These motors, however, need a controller/driver to process the output of the Hall effect devices. As disk drives have continued to decease in size and cost, the prohibited size and cost of the Hall effect devices has driven the need for a different solution for spindle motors.
Yet another solution for driving disk drives is the Hall-less DC brushless motor also called the back-EMF commutated brushless motor. This motor uses the concept that it is possible to deduce the location of the rotor of a brushless DC motor by monitoring the back-EMF voltage the motor generates. Every coil generates back-EMF as the motor spins. The back-EMF may be determined easily by measuring the voltage across a nondriven coil. The back-EMF voltage is directly proportional to the rotation rate of the motor. Therefore, at start-up time and at low rotation rates there is no back-EMF voltage with which to deduce commutation information. The problem with how to start the back-EMF commutated brushless motor has been addressed in several ways: one is to not address the problem and blindly energize coils in a particular order until rotation rate is sufficient to provide enough back-EMF to locate the rotational position of the rotor. This method suffers because frequently the rotor will be in a position at startup such that the torque generated will be the incorrect polarity and the disk will spin backwards. The back rotation is only for a short time and the motor will quickly get in synchronization and the disk will spin forward. However the back rotation is undesirable because the amount of wear between the recording head and the disk is in a direction for which the two were not designed. Furthermore, having the disk move in the opposite direction may dislodge particulates that have collected near the head-disk contact. These loose particulates may cause data errors and therefore pose a reliability problem. A second type solution is detailed in U.S. Pat. No. 4,876,491 entitled "Method and Apparatus for Brushless DC Motor Speed Control" by Squires et al. and U.S. Pat. No. 5,117,165 entitled "Closed-Loop Control of a Brushless DC Motor From Standstill to Medium Speed" by Cassat et al. Both these techniques make use of the fact that the inductance of a magnetic system is a function of the magnetic field through the system. Looking at FIG. 1, when the magnetic field through a piece of material is low the slope of the B/H curve (the inductance) is high. At higher magnetic field biases the slope of the B/H curve is flatter and the resulting inductance lower. By driving pulses into the phases of a back-EMF brushless DC motor and measuring the amplitudes of the resulting signals on the motor coils, it is possible to deduce the rotor position of the motor. However, this solution requires special apparatus for the generation of the pulses or small sinusoidal currents. Also, special hardware is required to measure the smaller than normal running motor amplitude of the resulting signals and a sequencer to supervise the process. This method applies enough current to get a reading, but not enough current to move the motor. The rotor position is sensed as a difference between these readings which will be small.
The limitations in the current art generated the need for a start-up commutation method for a back-EMF DC brushless motor without back rotation. Therefore, it is an object of this invention to provide an apparatus and method for starting up a back-EMF commutated brushless motor without the need for additional special motor drive apparatus. Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with the drawings herein.